A syngas generator is a device than can convert a fuel into a gas stream containing hydrogen (H2) and carbon monoxide (CO), commonly referred to as syngas. The syngas generator can be employed in an internal combustion engine exhaust after-treatment system used for emissions reduction. In this application the product syngas from the generator can be used to regenerate and/or heat one or more exhaust after-treatment devices such as Lean NOx Traps (LNT), Diesel Particulate Filters (DPF), and Diesel Oxidation Catalysts (DOC). There can also be other uses for syngas within the engine system.
For mobile applications, size, weight, reactant supply, durability and operating characteristics are some of the considerations when selecting the fuel conversion method. Partial oxidation reforming (POX) or auto thermal reforming (ATR) methods are both suited for mobile applications. An advantage of POX or ATR types of syngas generator is that the engine fuel and engine exhaust stream can be used as the reactants for the fuel conversion process, eliminating the need to carry or create an additional on-board reactant supply. POX or ATR syngas generators can be non-catalytic or catalytic. The non-catalytic type of POX or ATR syngas generators offer additional advantages such as increased durability (no catalyst to poison), reduced capital cost, fast response time, greater operating temperature range and the ability to withstand thermal cycles with larger magnitudes. The engine exhaust stream contains oxygen (O2), water (H2O), carbon dioxide (CO2) and heat, which can be utilized in the production of syngas. However in this situation, the reactant supply and composition will vary over the operating duty cycle of the engine, and this can present some challenges as described below. Furthermore, the pressure of the engine exhaust stream is generally limited.
Reforming of hydrocarbon fuel, especially heavy hydrocarbons (such as diesel), can be difficult due to the range of components that make up the fuel. These various components can react at different temperatures and rates. Inadequate vaporization and mixing of the fuel with the engine exhaust stream can lead to localized fuel-rich conditions, resulting in the formation of carbon within the syngas generator. Chemical decomposition of the hydrocarbon fuel can also lead to formation of carbon and residues, and can start at temperatures as low as 160° C. Carbon formation and removal are affected by the oxygen-to-carbon (O/C) and steam-to-carbon (S/C) ratios in the syngas generator. As mentioned above, the use of the engine exhaust stream as a reactant in the syngas generator imposes inherent supply and operational challenges. Operating with low and varying concentrations of oxygen, water, carbon dioxide and heat, at the same time as endeavoring to maintain appropriate oxygen-to-carbon (O/C) and steam-to-carbon (S/C) ratios to prevent detrimental carbon build-up over the operating duty cycle of the engine, is a challenge.
While many have attempted to eliminate or minimize carbon formation, practically there is an inevitable tendency for carbon to form during the conversion process of the fuel into syngas. Over time, carbon accumulation can impede the flow of gases, increase the pressure drop across the syngas generator, and reduce the durability of the syngas generator. Large accumulations of carbon also have the potential to create excessive amounts of heat that can damage the syngas generator if the carbon is oxidized in a short period of time. The carbon can also travel downstream of the syngas generator, increasing the back pressure of the engine exhaust and adversely affecting exhaust after-treatment devices, for example, by blocking catalyst and adsorbent reactive sites.
A non-catalytic syngas generator converts the fuel into syngas by cracking and reforming the fuel. This is an endothermic reaction and occurs at temperatures typically in the range of 600°-1400° C. The reaction temperature is dependent on various things such as: the hydrocarbon fuel being used, fuel conversion efficiency, and the degree of coke or soot (carbon) formation. A portion of the fuel can be combusted with oxygen in the engine exhaust stream to produce the required heat for the reaction. The O/C ratio of the fuel and engine exhaust mixture will affect the amount of heat produced and the temperature of the syngas generator. At these elevated temperatures, high thermal stresses can be created during the thermal cycling of the syngas generator. Thermal stresses can reduce the durability of the syngas generator over its lifetime.
Prior approaches to produce syngas include the use of catalytic POX or ATR fuel processors. The shortcomings of catalytic fuel processors include:                (1) Sintering of the catalyst if the catalyst is exposed to elevated temperatures. This can reduce the fuel conversion efficiency.        (2) Fatiguing of catalyst support materials during large thermal cycles. Small magnitude thermal cycles are desired, for example, less than a 100° C. range, to promote material durability.        (3) High capital cost resulting from the high cost of the catalyst material.        (4) Low durability as the catalyst is susceptible to poisoning by contaminants contained in the reactants.        
When syngas is used in the regeneration or heating of exhaust after-treatment devices in an internal combustion engine system, typically the requirement for syngas fluctuates or is intermittent. The requirement for syngas can be dependent, for example, on the engine exhaust emission output, the capacity of the after-treatment devices, the regeneration cycle of the after-treatment devices, the temperature of the exhaust gas and the heat loss of the exhaust system. When syngas is required, the syngas generator must typically be at or above a certain temperature in order to generate the required amount of syngas rapidly (typically within a few seconds) when it is needed. It is not practical to repeatedly start up and shut down the syngas generator. This would result in the thermal cycling of the syngas generator, which can reduce the durability and reliability of the device, as well as causing delays in the production of syngas. Furthermore, dynamically responsive start up and shut down of the syngas generator generally requires a fairly complex control system which can increase the cost of the system.
Prior approaches to the challenge of keeping the syngas generator warm during times when there is little or no requirement for syngas in the system have involved:                (1) Operating the syngas generator in a continuous, syngas-generating steady state mode, but discarding or diverting excess syngas to the inlet of the engine or to other syngas-consuming devices in the system. Discarding excess syngas results in a significant fuel penalty that will typically cause the overall efficiency of the engine system to be unacceptably low, and the operating cost to be high. Adding syngas to the fuel intake of an internal combustion engine utilizes the heating value in the syngas and can reduce NOx emissions. However, this approach can alter the combustion characteristics of the engine, requiring extensive testing prior to approval for use and warranty by the engine manufacturers.        (2) Reducing the supply of reactants (for example, fuel and engine exhaust) to the syngas generator so that it remains sufficiently warm that it can restart within a few seconds, but in the meantime produces smaller quantities of syngas. This approach increases the complexity of the reactant control which increases the cost of the exhaust after-treatment system.        (3) Using a “mini” syngas generator (such as a catalytic partial oxidizer) to feed the main syngas generator sufficiently to keep it warm, so it can be rapidly restarted. In this approach, reduced amounts of reactants are mixed and applied to a continuously-running smaller syngas generator, the output of which is applied to the main syngas generator that supplies syngas to the after-treatment system. The output of the smaller syngas generator provides heat as well as an easily lighted fuel that can be used to maintain the main syngas generator at a near-operating temperature. However, this requires a second fuel processor device in the system which adds to the system complexity and cost.        
Prior approaches to maintain the operating temperature of a catalytic type fuel processor within a desired range have involved supplying fuel to the fuel processor in a pulsed, discontinuous manner (time based) to provide alternating rich and lean periods, even during syngas production. During the lean periods, the fuel flow can be shut-off or set at a sub-stoichiometric (stoichiometry<1) flow rate. During the rich periods, an excess amount of fuel (stoichiometry>1) is supplied. Alternatively, the fuel processor can be supplied with fuel in essentially a continuous manner but only to a portion of the catalyst bed at a given time (spatial based) forming alternating rich and lean zones. At a given time, the portion of the catalyst bed receiving the fuel will be a rich zone, while the portion of the catalyst bed that receives no or a limited amount of fuel (with a stoichiometry of less than 1), will be a lean zone. A negligible or a limited amount of heat is produced under lean conditions which limits the temperature rise of the catalysts and fuel processor. The alternating cycle (fuel flow rate, duration and frequency) between the rich and lean periods or zones are selected to maintain the magnitude of the thermal cycles at a desired level.
The shortcoming of a catalytic type fuel processor and the above approach to maintain the desired operating temperature of the fuel processor can include:                (1) High frequency cycling of the fuel supply is required, for example, less than or equal to 10 seconds to prevent the temperature of the catalyst from moving outside a desired range and to reduce the magnitude of the thermal cycles of the catalyst.        (2) Higher operating fuel penalty.        (3) Increased component and control complexity and cost.        
The present approach overcomes at least some of the shortcomings of these prior approaches and offers additional advantages.